The present invention relates to a low pressure mercury discharge lamp which is a fluorescent lamp having excellent color rendering property to impart a color of a lightened object similar to that lightened by natural light.
In the specification, the fluorescent material means "phosphor" and the boron phosphate means "borate-phosphate" and the bright line spectrum means "line spectrum" and the luminous layer means "fluorescent layer".
The color rendering property of a fluorescent lamp is evaluated depending upon shape of a spectral distribution.
Accordingly, as one manner for improving a color rendering property of a fluorescent lamp, it has been proposed to combine various fluorescent materials having different emission spectra whereby the spectral distribution is varied to obtain a desired color rendering property.
However, radiation from a fluorescent lamp usually comprises a continuous spectrum part emitted from the fluorescent layer and a bright line spectrum part emitted from a mercury vapor discharge.
The mercury bright line spectrum prevents an improvement of the color rendering property. The kinds of fluorescent materials which can be applied to a commercial fluorescent lamp are limited in practice whereby the spectral distributions which can be given are limited. Accordingly, improvement of the color rendering property is limited.
In order to improve the color rendering property, it has been proposed to control blue mercury bright line spectrum at 405 nm and 436 nm which cause deterioration of the color rendering property. It has been proposed to provide a two layer coating system wherein a first layer for absorbing blue light is formed on an inner surface of a discharge tube by coating a red emitting fluorescent material for absorbing blue light to convert the energy to red light, and a second layer of a fluorescent material for emitting in blue, green, yellow and orange wavelength region is coated on the first layer. In this method, magnesium lithium arsenate activated by a tetravalent manganese compound or magnesium fluorogermanate are used as the fluorescent material in the first layer. These fluorescent materials form a blue light absorbing layer and additionally supply red light.
As the results of these studies, fluorescent lamps having good color rendering property have been obtained. However, these fluorescent lamps in the two layer coating system have serious disadvantages as follows. The red fluorescent material activated by tetravalent manganese has low luminous efficiency. The visible light emitted from the second layer is partially absorbed by the first layer and only a small amount of the absorbed energy is emitted again on the visible wavelength region, whereby the luminous efficiency of the fluorescent lamp is seriously decreased disadvantageously.
It has been also proposed to substitute the fluorescent material in the first fluorescent layer with a yellow pigment such as Titan yellow which absorbs blue light. However, in this method, the visible light emitted from the second fluorescent layer is also absorbed whereby the luminous efficiency is decreased.
These two layer coating systems have further serious disadvantages. Since it has required two coating steps, the processing time is increased and the control of light color and color rendering property is relatively difficult.